1. Field of the Invention
The present invention relates to signal separators, particularly teletext-signal separators, including a sampling device which samples a signal bit sequence received with a transport signal at a sampling frequency higher than the frequency of the signal bit sequence, and a processing device for recovering the signal bit sequence from the sampled values.
2. Description of the Related Art
The following describes a processing device for recovering a teletext bit sequence from the sample values of a teletext bit sequence received with a television signal, with the teletext signal being sampled at a sampling frequency higher than the frequency of the teletext bit sequence. However, the television signal only represents a special case of a transport signal and the teletext bit sequence represents a special case of a signal bit sequence transported with the transport signal.
Whenever a signal bit sequence is sampled at a sampling frequency which is not an integral multiple of the signal frequency, and/or if very few sample values are supplied per signal bit, a signal separator of the type described herein can be used to improve the reliability of the estimation of the values of signal bits.
Since the introduction of teletext ("videotext"), television sets equipped with suitable decoders can receive additional information. This information is transmitted simultaneously with the picture in a normal television channel. It is displayed in the form of information pages which may consist of text and graphics and are transmitted line by line during the vertical blanking interval. The teletext data, which are transmitted with the composite picture signal, must be converted by the decoder into an error-free digital data stream. The structure of such teletext data lines is shown in FIG. 1 A teletext data line begins with a number of clock synchronization pulses, the so-called clock run-in (CRI), which are followed by a "framing code" (FC). These signal components, which serve synchronization purposes, are followed by data (DA) which are combined in 8-bit data words. The clock run-in consists of a two-byte long sequence of alternating 0's and 1's which represents the highest frequency occurring in the teletext signal. The clock run-in enables the teletext-signal separator, which is part of the teletext decoder, to synchronize with the bit stream. The framing code is one byte long and serves, among other things, to recognize the byte boundaries within a teletext signal.
The teletext signal, which has a bit frequency that depends on the respective teletext standard, is sampled at a clock frequency of the television set. The frequency at which the teletext signal is sampled is not linked to the teletext bit frequency. In a known chrominance-subcarrier-locked sampling system, the sampling frequency is, for example, four times the chrominance-subcarrier frequency. The teletext bit frequencies used are quite high, so that in the NTSC or PAL system, the sampling frequency is only about 2.5 times the bit frequency. Thus, at least in the case of these television standards, the sampling frequency used for digitizing the teletext signal is not an integral multiple of the teletext bit frequency used at the transmitting end. This means that the sampling instants of the teletext signal are not phase-locked to the teletext signal, but that, if the center of the teletext bit is taken as a reference, the center and the sampling instants drift relative to each other. Thus, in the case of successive teletext bits, the sampling instants are not always located at the same point of a teletext bit but move, for example, from one edge via the top to the second edge. The same applies analogously to real teletext signals, which may deviate considerably from the square shape. For example, the teletext signals may assume an approximately sinusoidal waveform. At the frequency ratio being considered, not more than 2.5 sample values are available on the average for the estimation of the binary values of the individual teletext bits. That is, not more than 2.5 sample values are available for the determination as to whether the current teletext bit is a logic 0 or 1, so that reliable recovery of the teletext signal is not automatically ensured.
Prior attempts to make such an estimation have included the method of selecting only a part of the successive sample values, namely alternately the second and third sample values (2-3 rhythm), which are each regarded as being representative of the value of a teletext bit. However, it has been found that noise on the teletext signal may result in a false beginning of this alternating selection, and thus, in an incorrect estimation of the values of the teletext bits. Since in the prior art circuit, the sampling frequency was chosen to be only approximately 2.5 times the teletext bit frequency, the safe assignment of the sample values selected in accordance with the 2-3 rhythm to the individual teletext bits is lost in the course of time.
In a television set, the tuner is followed by a band-pass filter which filters the desired signal out of the received frequency spectrum. If the tuner is not correctly tuned, the teletext signal may be distorted, which, in the case of the aforementioned 2-3 rhythm method, may result in an incorrect determination of the values of the teletext bits. It is the object of the present invention to improve the reliability of the estimation of the values of signal bits.